Composite membrane containing a cross-linked enzyme matrix for a biosensor

ABSTRACT

An electrochemical sensor system and membrane and method thereof for increased accuracy and effective life of electrochemical and enzyme sensors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 09/872,240, filed on May 31, 2001, now U.S. Pat.No. 6,960,466 the entire disclosure of which is incorporated byreference herein.

FIELD OF THE INVENTION

The present invention is related to the field of electrochemicalsensors, particularly enzyme-electrode sensors, and to the regenerationor maintenance of the functional properties of the membranes of suchsensors.

BACKGROUND OF THE INVENTION

In a variety of clinical situations it is important to measure certainchemical characteristics of the patient's blood such as pH, hematocrit,the ion concentration of calcium, potassium, chloride, sodium, glucose,lactate, creatinine, creatine, urea, the partial pressure of O₂, andCO₂, and the like. These situations range from a routine visit of apatient in a physician's office to monitoring of a patient duringopen-heart surgery. The required speed, accuracy, and other performancecharacteristics vary with each situation.

Typically, electrochemical sensor systems which provide blood chemistryanalysis are stand-alone machines or are adapted to be connected to anextracorporeal shunt or an ex vivo blood source such as a heart/lungmachine used to sustain a patient during surgery. Thus, for example,small test samples of ex vivo blood can be diverted off-line from eitherthe venous or arterial flow lines of a heart/lung machine directly to achamber exposed to a bank of micro-electrodes which generate electricalsignals proportional to chemical characteristics of the real timeflowing blood sample.

Electrochemical sensor systems are analytical tools combining a chemicalor biochemical recognition component (e.g., an enzyme) with a physicaltransducer such as a platinum electrode. The chemical or biochemicalrecognition component is capable of selectively interacting with ananalyte of interest and of generating, directly or indirectly, anelectrical signal through the transducer. Electrochemical sensor systemsplay an increasing role in solving analytical and clinical problems, andfind applications in the field of medical diagnostics.

The selectivity of certain biochemical recognition components makes itpossible to develop electrochemical sensors which can accurately detectcertain biological analytes even in a complex analyte mixture such aswhole blood. Despite the high degree of selectivity of certainbiochemical recognition components, the selectivity of such sensors as awhole may nonetheless be compromised by the presence of certainbiological interferents (e.g. ascorbic acid, uric acid, acetaminophen,cysteine, etc.) which can directly interact with the physical transducerif they are not prevented from doing so. Accuracy and precision ofelectrochemical sensor systems with biochemical recognition compounds isalso compromised by residual levels of analyte remaining in the sensorfrom a prior sample affecting the analysis of the following sample.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a system and methodfor increasing the accuracy and effective lifetime of an electrochemicalsensor. Polymerization of electropolymerizable monomers into an innerpolymeric membrane on the electrochemical sensor forms an interferencerejection membrane. This inner polymeric membrane functions to protectthe electrochemical sensor from the fouling or interference by compoundsin the sample and thus increase the accuracy that is lost by the foulingdegradation of the membrane or by interference by analyte compounds fromthe sample.

In one aspect of the present invention, an electrochemical sensorincludes at least one electrode, and a composite membrane. The compositemembrane includes an outer layer, an enzyme layer, and a restorableinner layer. The inner layer is in contact with at least one electrodeand includes a polymerizable membrane.

The outer layer of the composite membrane may include a compoundselected from the group consisting of polyurethane-based compounds,polyvinyl-based compounds, silicone elastomer-based compounds, andpolycarbonate-based compounds. In one embodiment, the enzyme layer ofthe electrochemical sensor includes a H₂O₂ generating enzyme, such asglucose oxidase or lactate oxidase, for example. In another embodiment,the enzyme layer includes one or a combination of several enzymes, suchas a mixture of glucose oxidase, lactate oxidase, creatininase,creatinase, and sarcosine oxidase. In one embodiment, theelectrochemical sensor further includes a restored surface on the innerlayer wherein the surface is restored by polymerized monomer. The innerlayer of the electrochemical sensor may include a compound selected fromthe group consisting of benzothiophene, phenylenediamines, anddihydroxybenzenes.

In one aspect of the present invention, an electrochemical sensorcartridge, includes an electrochemical sensor card, at least oneelectrochemical sensor, and a reservoir containing anelectropolymerizable monomer solution in fluid communication with theelectrochemical sensor card.

In an embodiment of the present invention, the electrochemical sensorcartridge may include an electrochemical sensor card that includes atleast one composite membrane. In another embodiment, the electrochemicalsensor cartridge may include a composite membrane with a restorableinner layer.

In an embodiment of the present invention, the electrochemical sensorcartridge includes at least one calibration solution reservoir in fluidcommunication with the electrochemical sensor card. In anotherembodiment the electropolymerizable monomer solution may be combinedwith the calibration solution in a single reservoir. In anotherembodiment of the present invention, the electrochemical sensorcartridge includes electropolymerizable monomer solution in thecalibration solution wherein the concentration of the monomer is in therange of about 1-100 mM.

In another embodiment, at least one of the electrochemical sensors ofthe electrochemical sensor cartridge comprises an enzyme electrodesensor. In another embodiment the electrochemical sensor of theelectrochemical sensor cartridge is formed on an electrode composed froma material selected from a group consisting of platinum, gold, carbon orone of their modified structure. In another embodiment theelectrochemical sensor includes an electropolymerizable monomer selectedfrom a group consisting of benzothiophene, phenylenediamines, anddihydroxybenzenes. In another embodiment the electrochemical sensor isselective for a hydrogen ion, carbon dioxide, oxygen, sodium ion,potassium ion, ionized calcium, chloride, hematocrit, glucose, lactate,creatine, creatinine or urea. In yet another embodiment, theelectrochemical sensor includes a electropolymerizable monomer that is aderivative of phenylenediamine.

In another aspect of the present invention, an electrochemical sensorsystem includes an electrochemical sensor card including at least oneelectrochemical sensor, wherein the electrochemical sensor includes atleast one polymeric membrane. The electrochemical sensor system alsoincludes an electrochemical sensor apparatus that is in electricalcontact with the electrochemical sensor card. The electrochemical sensorapparatus is configured to measure electrical signals from theelectrochemical sensor card and is capable of providing an electricalpotential to the electrochemical sensor for the polymerization of theelectropolymerizable monomer solution to the polymeric membrane. Theelectrochemical sensor system also includes a reservoir containing anelectropolymerizable monomer solution in fluid communication with theelectrochemical sensor card. The electropolymerizable monomer solutionis polymerized to the polymeric membrane by the electrical potentialprovided by the electrochemical sensor apparatus.

In an embodiment of the present invention, the electrochemical sensorcartridge may include an electrochemical sensor card that includes atleast one composite membrane. In another embodiment, the electrochemicalsensor cartridge may include a composite membrane with a restorableinner layer.

In an embodiment, the electrochemical sensor system further includes acalibration solution in a reservoir in combination with anelectropolymerizable monomer solution. The concentration of theelectropolymerizable monomer solution is in the range of about 1-100 mM.In another embodiment, the electrochemical sensor system includes atleast one enzyme electrode sensor. In yet another embodiment, theelectrochemical sensor system includes an electrochemical sensor that isselective for a compound selected from a group consisting of hydrogenion, carbon dioxide, oxygen, sodium ion, potassium ion, ionized calcium,chloride, hematocrit, glucose, lactate, creatine, creatinine or urea.

In yet another embodiment, the electrochemical sensor system includes anelectropolymerizable monomer that is selected from a group consisting ofbenzothiophene, phenylenediamines, and dihydroxybenzenes, of which theconcentration of the electropolymerizable monomer solution in thecalibration solution is 1-100 mM. In another embodiment, electrochemicalsensor system includes an electrochemical sensor apparatus capable ofproviding an electrical potential for at least the partial removal ofinterfering agents in the polymeric membrane. In another embodiment,electrochemical sensor system further includes an outer membrane and anenzyme layer, in which the enzyme layer is in contact with the outermembrane and the polymeric membrane.

In another aspect, the invention relates to accelerating the recovery ofthe electrochemical sensor during the rinse process following exposureto a sample so that the recovery time of the electrochemical sensorsystem in a shorter time period. The reduction in recovery time isaccomplished by removing interfering agents from the polymeric membranelayer. Residual concentration of substrates for the enzymatic reactionand the products of the enzymatic reaction after exposure of theelectrochemical sensor to a sample, are examples of interfering agents.Another example of interfering agents is the residual concentration ofthe electropolymerizable monomer in the polymeric membrane afterexposure of the electrochemical sensor to the electropolymerizablemonomer solution.

The removal of interfering agents from a polymeric membrane isaccomplished by providing an electrochemical sensor including anelectrode and a composite membrane, the composite membrane including atleast one polymeric membrane, an electrical source in electrical contactwith said electrode, and by applying an electrical potential to theelectrode sufficient to cause at least a portion of the interferingagents in the polymeric membrane in contact with the electrode to beremoved. In one embodiment, the electrical potential is in a range ofabout 0.1 to 0.8 V versus the on-board reference electrode and isapplied for a range of time from about 10 to 200 seconds. In anotherembodiment, the electrical potential is about 0.4 V versus the on-boardreference electrode and is applied for about 50 seconds.

In another aspect, the invention relates to the method of restoring thefunctional properties of an electrochemical sensor. The method includesproviding an electrochemical system, which includes an electrochemicalsensor card including at least one electrochemical sensor. Theelectrochemical sensor includes an electrode and a composite membrane,the composite membrane including at least one polymeric membrane. Theelectrochemical sensor system also includes an electrochemical sensorapparatus in electrical contact with the electrochemical sensor card.The electrochemical sensor apparatus is configured to measure electricalsignals from the electrochemical sensor card and to provide anelectrical potential to the electrochemical sensor. The electrochemicalsensor system also includes a reservoir containing anelectropolymerizable monomer in a solution in fluid communication withthe electrochemical sensor card. The electropolymerizable monomersolution is polymerized to the polymeric membrane by the electricalpotential provided by the electrochemical sensor apparatus. The methodof restoring the functional properties of an electrochemical sensor alsoincludes contacting the electrochemical sensor with the solution andapplying an electrical potential of sufficient strength and sufficientduration to cause at least a portion of the electropolymerizable monomerin the solution to polymerize onto the polymeric membrane.

In an embodiment, the method of restoring the functional properties ofan electrochemical sensor includes adding the electropolymerizablemonomer to a calibrating solution to form the electropolymerizablemonomer solution. In one embodiment, the electrical potential comprisesa range of about 0.1 to 0.8 V versus the on-board reference electrodeand is applied for a range of time from about 30 seconds to 1 hour. Inanother embodiment, the electrical potential comprises about 0.5 Vversus an on-board reference electrode and is applied for about 3minutes.

In an embodiment, the method of restoring the functional properties ofan electrochemical sensor further includes the step of applying anadditional electrical potential of sufficient strength and sufficientduration to the electrode to cause removal of at least a portion ofinterfering agents in the polymeric membrane. In one embodiment, theelectrical potential is in a range of about 0.1 to 0.8 V versus theon-board reference electrode and is applied for a range of time fromabout 10 to 200 seconds.

In another aspect, the invention relates to the method for restoring thefunctional properties of an electrochemical sensor cartridge. The methodincludes the steps of connecting an electrochemical sensor cartridgethat includes an electrochemical sensor to an electrochemical sensorapparatus. The electrochemical sensor includes an electrode and acomposite membrane, which includes at least one polymeric membrane. Themethod further includes contacting the electrochemical sensor withelectropolymerizable monomer solution from the cartridge, and applyingan electrical potential of sufficient strength and sufficient durationto cause at least a portion of the electropolymerizable monomer solutionto polymerize onto a polymeric membrane. In one embodiment, the methodfurther includes adding an electropolymerizable monomer to a calibratingsolution to form the electropolymerizable monomer solution. In aparticular embodiment, an electrical potential is applied at a range ofabout 0.1 to 0.8 V versus the on-board reference electrode. Theelectrical potential may be applied for a range of time from about 30seconds to 1 hour. In one embodiment, the method also includes applyingan additional electrical potential of sufficient strength and sufficientduration to the electrode to cause removal of at least a portion ofinterfering agents in the polymeric membrane. In one embodiment, theelectrical potential is in a range of about 0.1 to 0.8 V versus theon-board reference electrode and is applied for a range of time fromabout 10 to 200 seconds.

In another aspect, the invention relates to a composite membrane for abiosensor. The biosensor includes an inner membrane layer, an outermembrane layer, and an enzyme layer. The enzyme layer includes a matrixthat includes at least one enzyme, a cross-linking agent, and an enzymestabilizer. In one embodiment of the present invention, the compositemembrane includes one or more of the enzymes lactate oxidase,creatinase, sarcosine oxidase, and creatininase. In another aspect, theinvention relates to a matrix for an enzyme sensor. The matrix includeslactate oxidase, a cross-linking agent, and a enzyme stabilizer. In oneembodiment, the matrix forms a cross-linked matrix of proteins havingenzymatic activity. The matrix may form an electrochemical electrode.The matrix may also include bovine serum albumin. Other inert proteinssimilar to bovine serum albumin may also be included. In anotherembodiment, one or more of the cross-linking agent present in the matrixmay include a dialdehyde, glutaraldehyde, for example, a diisocyanato,1,4-diisocyanatobutane, for example, and a diepoxide,1,2,7,8-diepoxyoctane and 1,2,9,10-diepoxydecane, as examples. Inanother embodiment, the cross-linking agent present in the matrix is1-10% glutaraldehyde by weight. In yet another embodiment, thecross-linking agent present in the matrix is 5% glutaraldehyde byweight. In another embodiment, the enzyme stabilizer present in thematrix may include one or more of the compounds, polyethyleneimine,polypropyleneimine, poly(N-vinylimidazole), polyallylamine,polyvinylpiridine, polyvinylpyrollidone, polylysine, protamine and theirderivatives. In another embodiment, the enzyme stabilizer present in thematrix is 1-20% polyethyleneimine by weight. In another embodiment, theenzyme stabilizer present in the matrix is 5% polyethyleneimine byweight.

In yet another aspect, the invention relates to a matrix for an enzymesensor that includes creatinase, sarcosine oxidase, a cross-linkingagent and, an enzyme stabilizer. In one embodiment, the matrix alsoincludes creatininase. In one embodiment, the matrix forms across-linked matrix of proteins having enzymatic activity. The enzymesensor may form an electrochemical sensor. In another embodiment, one ormore of the cross-linking agent present in the matrix may include adialdehyde, glutaraldehyde, for example, a diisocyanato,1,4-diisocyanatobutane, for example, and a diepoxide,1,2,7,8-diepoxyoctane and 1,2,9,10-diepoxydecane, as examples. Inanother embodiment, the cross-linking agent present in the matrix is1-10% glutaraldehyde by weight. In yet another embodiment, thecross-linking agent present in the matrix is 5% glutaraldehyde byweight. In another embodiment, the enzyme stabilizer present in thematrix may include one or more of the compounds, polyethyleneimine,polypropyleneimine, poly(N-vinylimidazole), polyallylamine,polyvinylpiridine, polyvinylpyrollidone, polylysine, protamine and theirderivatives. In another embodiment, the enzyme stabilizer present in thematrix is 1-20% polyethyleneimine by weight. In another embodiment, theenzyme stabilizer present in the matrix is 5% polyethyleneimine byweight.

In yet another aspect, the invention relates to a matrix for an enzymesensor including one or more of the enzymes, lactate oxidase,creatinase, sarcosine oxidase and creatininase, a cross-linking agent,and an enzyme stabilizer.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and canexist in various combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and other objects, features and advantages of the presentinvention disclosed herein, as well as the invention itself, will bemore fully understood from the following description of preferredembodiments and claims, when read together with the accompanyingdrawings. The drawings are not necessarily to scale, emphasis insteadgenerally being placed upon illustrating the principles of theinvention.

FIG. 1 is a schematic diagram of the components of an electrochemicalsensor apparatus including a sensor cartridge with a bank of sensors anda thermal block for accelerated hydration and calibration of thesensors.

FIG. 2 illustrates a reverse frontal view of the sensor card, partlyfragmentary, of a cartridge embodiment of the invention.

FIGS. 3A-B illustrate cross-sectional views of an enzyme sensor.

FIG. 4 illustrates an embodiment of a pO₂ sensor.

FIG. 5 illustrates a frontal view of the electrode card contained in oneembodiment of the cartridge.

FIG. 6 illustrates cross-sectional views of an ion sensor.

FIGS. 7A-G illustrate the components of a thermal block assembly.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides electrodes and electrochemical sensorsystems for measuring characteristics of aqueous samples including, butnot limited to, blood, serum or other body fluids. Specifically, theinvention is directed to such sensors in which the electrodes include aninterference rejection membrane, which is the inner polymeric membraneof the composite membrane and is renewable in situ. The electrochemicalsensor systems according to the invention have increased accuracy andprecision and increased effective life spans. In preferred embodimentsof the invention, the sensor system is adapted to measure theconcentration or activity of blood gases (e.g., oxygen and carbondioxide) ions (e.g., sodium, chloride, potassium and calcium), glucose,lactate, creatine, creatinine, blood pH and hematocrit.

Definitions

In order to more clearly and concisely point out and describe thesubject matter which applicant regards as the invention, the followingdefinitions are provided for certain terms used in the followingdescription and claims.

As used herein, the term “electrode” refers to a component of anelectrochemical device which makes the interface between the externalelectrical conductor and the internal ionic medium. The internal ionicmedium, typically, is an aqueous solution with dissolved salts. Themedium may also comprise proteins in a stabilizing matrix.

Electrodes are of three types, working or indicator electrodes,reference electrodes, and counter electrodes. A working or indicatorelectrode measures a specific chemical species, such as an ion. Whenelectrical potentials are measured by a working electrode, the method istermed potentiometry. All ion-selective electrodes operate bypotentiometry. When current is measured by a working electrode, themethod is termed amperometry. Oxygen measurement is carried out byamperometry. Working electrodes may also function by including an enzymeas part of an enzyme layer that is part of a composite layer that is inclose contact with the electrode. The enzyme, which is specific to aparticular analyte, produces hydrogen peroxide, a by-product of thecatalytic reaction of the enzyme on the analyte. Hydrogen peroxide isdetected by the electrode and converted to an electrical signal. Areference electrode serves as an electrical reference point in anelectrochemical device against which electrical potentials are measuredand controlled. In one embodiment, silver-silver nitrate forms thereference electrodes. Other types of reference electrodes aremercury-mercurous chloride-potassium chloride or silver-silverchloride-potassium chloride. A counter electrode acts as a sink for thecurrent path.

As used herein, the term “sensor” is a device that responds tovariations in the concentration of a given chemical species, such asglucose or lactate, in a sample, such as a body fluid sample. Anelectrochemical sensor is a sensor that operates based on anelectrochemical principle and requires at least two electrodes. Forion-selective measurements, the two electrodes include an ion-selectiveelectrode and a reference electrode. Amperometric enzyme electrodesadditionally require a third electrode, a counter electrode. Moreover,enzyme sensors based on two electrodes, a working and referenceelectrode, are also common.

As used herein, the term “ion selective electrode” generally refers to asilver wire coated with silver chloride in contact with a buffersolution containing a chloride concentration (the inner solution). Thebuffer solution is covered with a polymeric ion-selective membrane thatis in contact with the test solution. The ion selective membranetypically consists of a high molecular weight PVC, a plasticizer, anionophore specific to a particular ion, and a borate salt. The surfaceof the polymeric membrane is in contact with the test sample on one sideand the inner buffer solution on the other side of the membrane.

As used herein, the term “dry electrochemical sensor” refers to the ionselective electrode, described above, and a reference electrode,described above. In the “dry chemical” embodiment, the ion-selectiveelectrodes have the same configuration as described above, however, theinner solution containing chloride, is dried, i.e., dehydrated leaving alayer of dry salt. In order to function as an electrochemical sensor,the dried salt must be solubilized in water to obtain a buffer solution.

As used herein, the term “enzyme electrode” generally refers to acomposite membrane deposited on a metal electrode, comprising platinumfor example. The composite membrane is at least three distinct layersincluding an outer polymeric membrane on the side of the compositemembrane in contact with the sample that forms a protective layer, amiddle enzyme layer that is located between the outer and inner layers,and an inner polymeric membrane closest to the metal electrode thatforms the inner interference rejection membrane. The outer polymericmembrane, which is comprised of one or more polymeric compounds,generally functions to protect and maintain the structure of the middleenzyme layer and to control the diffusion of the analyte into the middleenzyme layer. The middle or enzyme layer comprises at least one proteinspecies with enzymatic activity. The enzymatic activity may also beprovided by compounds which include DNA, RNA, and carbohydrate, forexample. The enzyme is stabilized in a matrix conducive to the activityof the enzyme. The inner or interference rejection membrane is apolymeric membrane that functions to insulate the wire electrode fromcompounds in the sample that interfere with the functioning and accuracyof the electrode.

As used herein, the term “hydration” refers to the process ofsolubilizing the salts of a sensor's inner salt layer by the passage ofwater through the ion-selective outer polymeric membrane bounding oneside of the inner salt layer, into the inner salt layer to form asolution. Hydration normally can be achieved by mere contact of theoutside of the polymeric membrane and inner salt solution with anaqueous salt solution for a required duration.

As used herein, “thermal cycling” is the process by which thetemperature of an electrochemical sensor, soaked in an aqueous saltsolution, is raised to a specified elevated temperature for a specifiedlength of time, and then lowered.

As used herein, the term “calibration” refers to the process by whichthe response characteristics of a sensor to a specific analyte aredetermined quantitatively. To calibrate a sensor, the sensor is exposedto at least two reagent samples, each reagent sample having a different,known concentration of an analyte. The responses, i.e., signals,measured by the sensor, relative to the concentrations of the analyte inthe two different reagent samples, serve as reference points formeasurements of the analyte in samples having unknown concentrations ofthe analyte.

Referring to FIG. 1, the electrochemical sensor system 8 employs asensor assembly, generally indicated at 10, incorporating a plurality ofelectrodes adapted to make electrical measurements on a sample; such asa blood sample, introduced to the sensor assembly 10. Blood samples tobe analyzed by the system are introduced through a sample inlet 13 a.Blood samples are obtained by, for example, phlebotomy or are derived ona periodic basis from an extracorporeal blood flow circuit connected toa patient during, for example, open heart surgery. Blood samples may beintroduced into the sample inlet 13 a through other automatic means, ormanually, as by syringe. The blood samples may be introduced as discretesamples.

The electrochemical system 8 including a number of essential componentsas heretofore described in a preferred embodiment of the presentinvention is contained in a disposable cartridge 37. A cartridge of asimilar type is set forth in detail in U.S. Pat. No. 4,734,184, theentirety of the specification incorporated by reference herein. In oneembodiment of the invention, the electrochemical sensor system 8incorporates in the cartridge 37 at least two prepackaged containers 14,and 16, each containing a calibrating aqueous solution having knownvalues of the parameters to be measured by the system. For purposes ofreference, the solution contained within the prepackaged container 14will be termed Calibrating Solution A, the solution contained within theprepackaged container 16 will be termed Calibrating Solution B. Inanother embodiment of the invention, the electrochemical system 8,illustrated in FIG. 1, includes a third prepackaged container 23containing Calibrating Solution AO. Each of the prepackaged containers14, 16 and 23 contain a sufficient quantity of its calibrating solutionto allow the system to be calibrated a substantial number of timesbefore the prepackaged container becomes empty. When one or more of thecontainers 14, 16 and 23 containing the calibrating solutions are empty,the cartridge containing prepackaged containers 14, 16 and 23 must bereplaced.

In a particular embodiment of the invention, the Calibrating Solution AOcontains electropolymerizable monomer. Electropolymerizable monomer suchas m-phenylenediamine may be included in the calibrating solutions at aconcentration in a range of about 1 to 100 mM, preferably about 15 mM.In another embodiment of the invention, a solution ofelectropolymerizable monomers is contained in a prepackaged container(not shown) separate from the prepackaged containers 14 and 16 for thecalibrated solutions at a concentration in a range of about 1 to 100 mM,preferably about 15 mM.

Referring to FIG. 1, in one embodiment the prepackaged container 14 isconnected to the input of a multi-position valve 18 through a flow line20, and the prepackaged container 16 is connected to a second input ofthe multi-position valve 18 through a flow line 22. In yet anotherembodiment, the container 23 is connected to a third input of themulti-position valve 18 through a flow line 25. Another container 17contains a rinse solution and is connected to the input of themulti-position valve 18 through a flow line 21. In yet anotherembodiment, the rinse bag 17 is eliminated and one of the calibrationsolutions A or B is used as a rinse solution, as well. The output line12 is the output of the multi-position valve 18 and is connected to thesample input line 13 through a stylus 11. Depending upon the position ofthe valve 18, the input lines 20, 21, 22, 25 or air is open to the valve18. Similarly, when the stylus is in a normal position (position 11 b)of the sample input line 13 b, line 12 b is open to the sample inputline 13 b and allows passage of the calibrating, or rinse solution, orair through the sample input line 13 b to the sensor assembly 10 throughline 24, facilitated by the operation of a peristaltic pumpschematically illustrated at 26. However, in a sample accepting mode (13a), line 12 is separated from the sample input line (position 12 a) andthe sample is introduced directly to the sensor assembly 10 through line24, facilitated by the operation of the peristaltic pump 26.

The cartridge 37 also includes a container 28, for a reference solution.The container 28 is connected to the sensor assembly by a flow line 30.The system further includes a container 32 for waste, which receives theblood samples, the calibrating solutions and the reference solutionafter they have passed through the sensor assembly 10, via a flexibleconduit 34 that has input from the sensor assembly 10.

Both the waste flow conduit 34 and the reference solution flow line 30consist of or include sections of flexible walled tubing that passthrough the peristaltic pump 26. The pump 26 compresses and strokes theflexible sections of the flow lines 30 and 34 to induce a pressured flowof reference solution from the container 28 to the electrode assembly 10and to create a negative pressure on the waste products in flow line 34so as to draw fluids, including the fluids with the polymerizablemonomers, in the flow line 24 through passages in the electrode assembly10 past the membranes of the sensors. This arrangement, as opposed tothe alternative of inducing positive pressure on the blood andcalibrating solutions to force them through the electrode assembly 10,avoids the imposition of unnecessary and possibly traumatic mechanicalforces on the blood sample and minimizes possibilities of leaks in theelectrode assembly 10.

Cartridge 37 also contains a sensor card 50 which provides a low volume,gas tight chamber in which the sample, such as a blood sample,calibration solution, or monomer-containing solution, is presented toone or more electrochemical sensors, i.e., the pH, pCO₂, pO₂, Na⁺, Ca⁺⁺,glucose, lactate, creatine, creatinine and hematocrit sensors, togetherwith the reference electrode collectively indicated as sensors 10, areintegral parts of the chamber. Chemically sensitive, hydrophobicmembranes typically formed from polymers, such as polyvinyl chloride,specific ionophores, and a suitable plasticizer, are permanently bondedto the chamber body. These chemically sensitive, hydrophobic membranes,described below in detail, are the interface between the sample orcalibrating solutions and the buffer solution in contact with the inner(silver/silver chloride) electrode.

In one embodiment of the invention, referring still to FIG. 1, includedin the cartridge 37, are three solutions that allow for calibrations athigh and low concentrations for all parameters except hematocrit, whichcalibrates at one level. In one embodiment, the cartridge 37 alsoincludes the rotor-for-sample inlet arm 5, the pump tubing 24, 30 and34, the sampling stylus 11, a waste bag 32, the reference solutioncontainer 28, the rinse solution container 17, calibration solutioncontainers 14, 16 and 23, the check valve 33, and tubes 12, 20, 21, 22and 25. Blood samples that have been analyzed are prevented from flowingback into the sensor card 50 from the waste container 32 due to thepresence of a one-way check 33 valve in the waste line 34. After use inthe system 8, the cartridge 37 is intended to be discarded and replacedby another cartridge.

Referring to FIG. 1, sensors are available as a bank of electrodes 10fabricated in a plastic card 50 and housed in the disposable cartridge37 that interfaces with a thermal block assembly 39 of a suitablyadapted blood chemistry analysis machine. The thermal block assembly 39houses the heating/cooling devices such as a resistive element or aPeltier-effect device, a thermistor 41 to monitor and control thetemperature, the electrical interface 38 between the sensors in theplastic card 50 and the microprocessor 40 through the analog board 45.The analog board 45 houses analog-to-digital and digital-to-analogconverters. The signal from the electrode interface 38 passes throughthe analog-to-digital converter, converted into digital form for theprocessor 40 to store and display. Conversely, the digital signals fromthe processor 40, for example, the polarization voltage for oxygensensor, go through the digital-to-analog converter, converted into ananalog form and fed to the sensors for control, through the electrodeinterface 38.

The electrochemical sensor system 8 is formed upon insertion of thecartridge 37 into the electrochemical sensor apparatus. Upon insertion,the sensor card 10 fits into the heater block assembly 39, described indetail below, and the heating/cooling assembly regulated by themicroprocessor 40 cycles the temperature of the sensor card 50 and thesolution in contact with the sensors inside the sensor card 50 through aspecific temperature for a specified duration. The heater block assembly39 is capable of rapid heating and cooling by, for example, athermoelectric device applying, for example, the Peltier-effect,monitored by a thermistor 41, all controlled by the microprocessor 40.The sensors connect to the electrode interface 38 which select one ofthe plurality of electrical signals generated by the sensors and passesthe electrical signal to the microprocessor 40 in the machine through ananalog-to-digital converter into the analog board 45 where it isconverted from analog to digital form, suitable for storage and display.Referring to FIG. 1, the electrode assembly 10 has a number of edgeconnectors 36 in a bank which allow it to be plugged into a femalematching connector 38 so that the electrodes formed on the assembly 10may be connected to microprocessor 40 through the analog board 45. Themicroprocessor 40 is connected to the multiport valve 18 via a valvedriver 43 by a line 42 and to the motor of the peristaltic pump 26 via apump driver 45 by a line 44. The microprocessor 40 controls the positionof the sample arm 5 through arm driver 15, and the position of the valve18 and the energization of the pump 26 to cause sequences of bloodsamples and calibrating solutions to be passed through the electrodeassembly 10. When the calibrating solutions from, for example,containers 14, 16 and 23 are pumped into the electrode assembly 10, theelectrodes forming part of the assembly make measurements of theparameters of the sample and the microprocessor 40 stores theseelectrical values. Based upon measurements made during the passage ofthe calibration solutions through the electrode assembly 10, and theknown values of the measured parameters contained within the calibratingsolution from containers 14, 16, and 23, the microprocessor 40effectively creates a calibration curve for each of the measuredparameters so that when a blood sample is passed through the electrodeassembly 10 the measurements made by the electrodes can be used toderive accurate measurements of the parameters of interest. Theseparameters are stored and displayed by the microprocessor 40. Themicroprocessor 40 is suitably programmed to perform measurement,calculation, storage, and control functions such as differences inelectrical potential across one or more electrodes.

Calibrating Solutions

In one embodiment of the invention a composition of calibrating solutionA used for second point calibration, prepared at 37° C. and atatmospheric pressure tonometered with 9% CO₂ 14% O₂ and 77% Helium gas,is as follows: pH 6.9 organic buffer; pCO₂=63 mmHg; pO₂=100 mmHg;Na⁺=100 mmol/L; K⁺=7 mmol/L; Ca⁺⁺=2.5 mmol/L; glucose=150 mg/dL;lactate=4 mmol/L; creatine=0.5 mmol/L; creatinine=0.5 mmol/L; surfactantand inert preservative.

In another embodiment of the invention a composition of calibrationsolution B used for one-point calibration and rinse, prepared at 37° C.and at 700 mmHg absolute pressure tonometered with 27% O₂, 5% CO₂, and68% Helium gas, is as follows: pH 7.40 organic buffer; pCO₂=34 mmHg;pO₂=180 mmHg; Na⁺=140 mmol/L; K⁺=3.5 mmol/L; Ca⁺⁺=1.0 mmol/L; surfactantand inert preservative.

In yet another embodiment of the invention a preferred composition ofcalibration solution AO for low level oxygen calibration and in situregeneration of the inner polymeric membrane for the enzyme sensorscontains aqueous solution of Na⁺, K⁺, Ca⁺⁺ salt; 15 mmol/L ofm-phenylenediamine, 20 mmol/L of sulfite, surfactant and inertpreservative; organic buffer, pCO₂. The reference solution containsAgNO₃=1 mmol/L; KNO₃=1 mol/L; surfactant.

The compositions of the A and B calibrating solutions are chosen so thatfor each of the characteristics measured by the system a pair of valuesare obtained that are spaced over the range of permissible values thatare measured by the system, providing a balanced 2-point calibration forthe instrument. The AO calibrating solution is chosen for low leveloxygen calibration and regeneration of the inner polymeric membrane inthe glucose, creatine, creatinine and lactate sensors.

The A and B calibration compositions are prepared by premixing all ofthe constituents in a certain order starting with the buffer and endingwith the sodium bicarbonate salt, then tonometering the solution withoxygen and CO₂ mixed with helium to produce the desired level of pCO₂and pO₂. The AO calibration solution is prepared with a slightdifference in procedure. The salts with the exception of sodium sulfite,m-phenylenediamine and sodium bicarbonate are added to water and thesolution is tonometered with helium to bring the pO₂ to less that 30mmHg. Then, the remaining salts are added to the solution and the finalmixture is tonometered with mixture of pCO₂ and helium to produce thedesired pCO₂ level.

At least one electropolymerizable monomer is added to at least one ofthe calibrating solutions, solution AO in container 23 for example. Theabsence of dissolved oxygen in the AO solution, due to presense ofsulfite ion, allows for a longer shelf life of electropolymerizablemonomer in the AO solution because dissolved oxygen will oxidize theelectropolymerizable monomer and thus render the monomer incapable ofpolymerizing. The electropolymerizable monomers m-phenylenediamine forexample, may be included in a calibrating solution at a concentration ina range between about 1 to 100 mM, preferably 15 mM. Theelectropolymerizable monomer may be included in the cartridge 37 in aseparate reservoir.

The temperature and pressure at which the calibrating solutions areprepared and their method of packaging must be such as to preclude thepossibility of dissolved gases going out of solution in the container,which would affect the concentration of gases in the calibratingsolutions, and to minimize the tendency for gases to permeate througheven the most impermeable materials practically obtainable. Thecalibration solutions are packaged with the solutions completely fillingthe containers, so that there is no head space, by evacuating thecontainers prior to filling in a manner which will be subsequentlydescribed.

By filling the calibration solution into the evacuated flexible wallcontainer 14, 16, 23 at elevated temperatures and subatmosphericpressure, the solution will not have any tendency at a lower usetemperature to outgas and thus produce gas bubbles in the container.Were outgassing to occur, the concentrations of the gases in thesolution would be affected, creating an inaccuracy in the calibration ofthe instruments. Similarly, the calibration solutions must not bepackaged at too low a pressure i.e., not below about 625 mm of mercury,because the absorptive capacity of the solution for gases conceivablyincreases as the packaging pressure decreases and below that pressurevalue the absorptive capacity of the solution may be sufficiently highthat it will tend to draw gases in through the slight inherentpermeability of even the most gas impervious flexible packagingmaterial, over long periods of time. Accordingly, a packaging pressurein the range of 625-700 mm of mercury is preferred.

In one embodiment, a calibrating solution prepared at a temperature inexcess of its intended use temperature so that at the lower temperaturethere is less tendency for outgassing of the dissolved gases. Thiscooperates with the reduced pressure packaging to minimize thepossibility of outgassing.

Calibration Solution A, B and AO are prepared at a temperature above itsintended use temperature at a controlled pressure close to atmosphericpressure. Through use of elevated temperature (e.g., 37° C.) thesolution may be prepared at about atmospheric pressure without anypossibility of subsequent microbubbles within the container or gastransfer through the container when packaged in a zero head spaceflexible gas impervious container.

The envelopes which form the calibration solution prepackaged containers14, 16, 23 are formed, for example, of rectangular sheets, heatsealed atthe edges and heatsealed at one corner to an inlet stem of the valve 18which is used for filling purposes. In the preferred embodimentillustrated, the prepackaged containers 14, 16, and 23 and theprepackaged container lines 20, 22, and 25 are formed in a unitarycluster with the valve 18 so that gas phase dead space in the lines 20,22, 25 is thereby avoided. In a preferred procedure for purging andfilling the envelope bags, the envelope is first evacuated and thenfilled with the prepared solution. The bag is then shaken while theexcess solution continually flows out of the bag. This process removesany residual gas bubbles from the bag. The solution is then sealed inthe container.

The calibration solutions in the prepackaged containers 14, 16, and 23have excellent stability and a long shelf life. When at use temperatureand atmospheric pressure there is no possibility of any outgassing fromthe liquid to form gas bubbles within the prepackaged containers 14, 16,and 23.

Reference Solution

The reference solution disposed in prepackaged container 28 is employedin the electrode assembly 10 as a supply source to a reference electrodeto provide a liquid junction and thereby isolate the reference electrodefrom the varying electrochemical potential of the calibrating solutionor the blood in a manner which will be subsequently described. In apreferred embodiment, the solution is 1 mol/L potassium nitrate and 1mmol/L silver nitrate solution. The solution also contains a surfactantsuch as Brij 35. The solution is packaged in a sealed flexible containerwith no head space.

Electrode Assembly

Referring to FIG. 1, during operation of the pump 26, the electrodeassembly 10 receives a constant pulsating flow of the reference solutionvia line 30 and sequential, intermittent pulsating flows of either theblood sample or one of the calibrating solutions via line 24. Theassembly also provides a corresponding output of its waste products to awaste collection bag 32 via line 34.

Referring to FIG. 2, by way of example, the electrode assembly 10 in apreferred embodiment consists of a structurally rigid rectangular card50 of polyvinylchloride having a rectangular aluminum (or other suitablematerial) cover plate 52 adhered to one of its surfaces. Cover plate 52closes off the flow channels 56 formed in one surface of the card 50 andalso acts as a heat transfer medium for hydrating the sensors by thermalcycling, described below, and to maintain the fluids flowing through theelectrode assembly 10, and the electrodes themselves, at a constanttemperature during calibration and during measurement of relevantparameters in a patient sample. This may be achieved by measuring thetemperature of the plate 52 and employing a suitable heating or coolingelement e.g., a Peltier-effect device and thermistor 41 to maintain thetemperature of the plate 52 at a desired temperature.

Referring to FIG. 2, a reference solution is introduced to a well 64,formed in the surface of the substrate 50 in the same manner as theother flow channels 56 and similarly covered by the metal plate 52. Thereference solution flow line 30 passes through an inclined hole in thewell 64. The well 64 is connected to the output section 34 of the flowchannel 56 through a very thin capillary section 66 formed in thesurface of the plastic substrate 50 in the same manner as the main flowchannels 56. The capillary channel 66 is substantially shallower andnarrower than the main flow channel 56; its cross section isapproximately 0.5 sq. mm. Reference fluid pumped into the well 64 by thepump 26, via a line 30 (see also FIG. 1), fills the well, and is forcedthrough the capillary section 66 where it joins the output stream offluid passing through the main flow channel section 56 and then flowswith it to the waste bag 32. The combined influence of its higherdensity described above and the capillarity of the flow channel 66serves to minimize any possibility of calibrating solution or bloodpassing downward through the channel 66 to the well 64 and upsetting theelectrochemical measurements.

As a blood sample or calibration solution quantity introduced into theflow channel 24 passes through the flow channel 56 to the output section34, it passes over a number of electrodes as illustrated in FIG. 2.

Referring to FIGS. 1 and 2, the heat plate 52 abuts and forms one wallof the sample channel 56. The heat plate 52 is in contact with thePeltier-effect device of the thermal block assembly 39 described below.The thermal block assembly 39 is capable of changing and controlling thetemperature of the heat plate 52 between 15° C. and 75° C. Thetemperature change and control is monitored by a thermistor 41 andregulated by the microprocessor 40. An internal digital clock of themicroprocessor 40 controls time and can switch on and switch off thethermal block assembly 39 according to a preset program. Thus,microprocessor 40 controls the thermal block assembly 39, regulating thetemperature setting and the duration of each set temperature of the heatplate 52.

The Electrodes

The order of assembly of the electrodes given below is only by way ofexample and is not intended to be limited to the order provided.

The Hematocrit Electrode Pair

Referring to FIG. 2, a pair of gold wires 98 and 100 form electrodes fordetermining the hematocrit (Hct) of a sample based on its conductivity.The wires make contact with printed circuit edge connectors 102 and 104,respectively, also illustrated in FIG. 5.

The Oxygen Sensor

Referring to FIG. 2, the next sensor in the flow channel 56 is theoxygen sensor 70 with a three electrode configuration, also illustratedin FIG. 4.

The Potassium, Calcium and Sodium Ion Sensing Electrode

Next up the flow channel is a sodium sensing electrode 78, followed by acalcium sensing electrode 86 and a potassium sensing electrode 90including an active membrane and a staked silver wire and an associatededge connector.

The pH Electrode

Referring to FIG. 2, next along the flow channel 56 is a pH sensingelectrode 94 also illustrated in FIG. 6 which includes a membrane 148and a silver wire 87 staked or press-fitted through the thickness of theplastic 50 into the flow channel 56. Referring to FIG. 6, joined on theopposite side of the flow channel 56 is a pad printed conductor section88 (also see FIG. 5) that forms an edge connector. The nature of this pHelectrode will be subsequently described in detail.

The Carbon Dioxide Electrode

Referring to FIG. 2, the next electrode 93 along the flow channel 56measures the dissolved carbon dioxide in the blood or calibratingsolution and works in combination with the pH electrode 94.

The Lactate Electrode

Referring to FIG. 2, next along the flow channel 56, lactate electrode92 functions by measuring by-products of an enzymatic reaction oflactate oxidase on lactate. The lactate oxidase present in the enzymelayer oxidizes the lactate producing hydrogen peroxide, which isdetected by the electrode of the lactate sensor.

The Glucose Electrode

Referring to FIG. 2, a glucose electrode 91 is the next electrode, whichlike the lactate electrode 92 functions by the detection of hydrogenperoxide produced by an enzymatic reaction in the enzyme layer. Theenzyme, glucose oxidase, specifically oxidizes glucose and produceshydrogen peroxide, a compound detected by the electrode of the glucosesensor.

The Creatine and Creatinine Electrodes:

Measurement of creatinine in a blood sample requires two electrodes. Oneelectrode measures the total concentration of creatinine and creatineand the other electrode measures the concentration of only creatine. Theconcentration of creatinine is determined by subtraction of creatinefrom the combined creatine and creatinine concentrations. Referring toFIG. 2, the next two electrodes, creatinine 116 and creatine 118, whichlike the glucose electrode 91 and lactate electrode 92, function bydetection of H₂O₂ produced by enzymatic reaction in their respectiveenzyme layers. In the creatinine electrode 116, the enzyme layerincludes a mixture of three enzymes: creatininase, creatinase andsarcosine oxidase. This enzyme mixture specifically oxidizes creatinineand creatine and produces H₂O₂ in the following cascade reaction.

In the creatine electrode 118, the enzyme layer includes a mixture oftwo enzymes: creatinase and sarcosine oxidase. This enzyme mixturespecifically oxidizes only creatine and produces H₂O₂ in the followingcascade reaction:

The Ground

The ground 105 illustrated in FIG. 2, is a silver wire inserted throughthe substrate 50. A ground serves as a common electric reference pointfor all electrodes. The ground may also serve as a counter electrode forthe amperometric sensor system.

The Reference Electrode

As illustrated in FIG. 2, two silver wires 106 are staked through thethickness of the plastic substrate board 50 into the reference solutionwell 64 to act as the on-board reference electrode. Use of two silverwires 106 which are electrically connected assures continuous contactbetween the silver wire and the reference solution in the presence ofair bubbles. Air bubbles may form in the reference channel as a resultof degassing the reference solution at the elevated temperature of thesensor control. A printed circuit element 108, also illustrated in FIG.5, extends along the back of the panel between the one end of thisreference electrode and edge of the board to provide an edge connector.

The specific construction and operation of the electrodes will now bedescribed in detail.

Specifics of Ion Selective Electrodes

The details of ion-selective electrodes are described, for example, inU.S. Pat. No. 4,214,968, incorporated by reference herein, and U.S. Pat.No. 4,734,184, incorporated by reference herein.

Ion-selective membranes of this type, which are also known as liquidmembranes, constitute a polymeric matrix with a non-volatile plasticizerwhich forms the liquid phase in which an ion carrier or selectorcommonly referred to as an ionophore, which imparts selectivity to themembrane, is dispersed.

Ion-Selective Membrane Polymer

Polymers for use in the ion-selective membrane of the instant inventioninclude any of the hydrophobic natural or synthetic polymers capable offorming thin films of sufficient permeability to produce, in combinationwith the ionophores and ionophore solvent(s), apparent ionic mobilitythereacross. Specifically, polyvinyl chloride, vinylidene chloride,acrylonitrile, polyurethanes (particularly aromatic polyurethanes),copolymers of polyvinyl chloride and polyvinylidene chloride, polyvinylbutyral, polyvinyl formal, polyvinylacetate, silicone elastomers, andcopolymers of polyvinyl alcohol, cellulose esters, polycarbonates,carboxylated polymers of polyvinyl chloride and mixtures and copolymersof such materials have been found useful. Films of such materials whichinclude the ionophores and plasticizers may be prepared usingconventional film coating or casting techniques and, as shown in theexamples below, may be formed either by coating and film formationdirectly over the internal reference electrode or some suitableinterlayer or by formation separately and lamination thereto.

Ionophore

The ionophore used in the ion-selective membrane is generally asubstance capable of selectively associating or binding to itselfpreferentially a desired specific alkali metal, alkaline earth, ammoniumor other ions. Suitable ionophores are more fully described below.

The selectivity of the electrode for a particular ion is due to thechemical nature of the ionophore and, thus, the use of differentchemical components as the ionophore provides different membranes foruse in ion-selective electrodes specific to different ions. Exemplary ofsuch components are a large number of substances, some of them known tobe antibiotics, which includes:

-   -   (1) valinomycin, a potassium-selective ionophore;    -   (2) cyclic polyethers of various constitution which make the        membrane selective to lithium, rubidium, potassium, cesium or        sodium; and    -   (3) other substances having ion selectivity similar to        valinomycin such as other substances of the valinomycin group,        tetralactones, macrolide actins (monactin, nonactin, dinactin,        trinactin), the enniatin group (enniatin A, B),        cyclohexadepsipeptides, gramicidine, nigericin, dianemycin,        nystatin, monensin, esters of monensin (especially methyl        monensin for sodium ion-selective membranes), antamamide, and        alamethicin (cyclic polypeptides).

Numerous other useful materials are described in the foregoingpublications and patents, as well as other literature on this subject.

The concentration of ionophore in the membrane will, of course, varywith the particular carrier used, the ion undergoing analysis, theplasticizer, etc. It has generally been found, however, that ionophoreconcentrations of below about 0.1 g/m² of membrane assuming the membranethicknesses preferred herein result in marginal and generallyundesirable responses. Ionophore concentrations of between about 0.3 andabout 0.5 g/m² are preferred. The ionophore can be incorporated atlevels much higher than this; however, because of the cost of many ofthese materials, use of such levels is not economically sound.

Plasticizer

The plasticizer provides ion mobility in the membrane and, the presenceof a plasticizer is necessary to obtain good ion transfer.

The plasticizer must, of course, be compatible with the membrane polymerand be a solvent for the ionophore.

The other highly desirable characteristic is that the plasticizer besufficiently insoluble in water that it does not migrate significantlyinto an aqueous sample contacted with the surface of the membrane asdescribed hereinafter. Generally, an upper solubility limit in waterwould be about 4.0 g/l with a preferred limit lying below about 1 g/l.Within these limits, substantially any solvent for the ionophore whichis also compatible with the polymer may be used. It is also desirablethat the ion plasticizer be substantially non-volatile to provideextended shelf-life for the electrode. Among the useful solvents arephthalates, sebacates, aromatic and aliphatic ethers, phosphates, mixedaromatic aliphatic phosphates, adipates, and mixtures thereof. Specificuseful plasticizers include trimellitates, bromophenyl phenyl ether,dimethylphthalate, dibutylphthalate, dioctylphenylphosphonate,bis(2-ethylhexyl)phthalate, octyldiphenyl phosphate, tritolyl phosphate,tris(3-phenoxyphenyl) phosphate, tris(2-ethylhexyl) phosphate, anddibutyl sebacate. Particularly preferred among this class arebromophenyl phenyl ether and trimellitates for potassium electrodesusing valinomycin as the carrier.

A large number of other useful plasticizers permit assembly ofelectrodes of the type described herein and may be used in thesuccessful practice of the instant invention.

The concentration of plasticizer in the membrane will also vary greatlywith the components of a given membrane; however, weight ratios ofplasticizer to polymer of between about 1:1 to about 5:2 provide usefulmembranes. The thickness of the membrane will affect electrode responseas described in somewhat more detail below, and it is preferred tomaintain the thickness of this layer below about 5 mils and preferablyabout 1 mil. As also described in greater detail below, the uniformityof thickness of the ion selective membrane plays an important role inthe optimum utilization of electrodes of the type described herein.Thus, if maximum advantage in terms of storage capability is to beobtained, the ion-selective membrane should be of relatively uniformthickness as defined above.

Support

Referring to FIG. 1, the electrodes of the present invention include asupport or card 50 which may be comprised of any material capable ofbearing, either directly or by virtue of some interveningadhesion-improving layer, the other necessary portions of the electrodewhich are described in detail hereinafter. Thus, the support maycomprise ceramic, wood, glass, metal, paper or cast, extruded or moldedplastic or polymeric materials, etc. The composition of the supportcarrying the overlying electrode components must be inert; i.e., it doesnot interfere with the indicating potentials observed as, for example,by reacting with one of the overlying materials in an uncontrolledfashion. Moreover, the composition of the support must withstandelevated temperatures to which the sensors will be exposed, for the timelength required to hydrate and/or calibrate the sensors. In the case ofporous materials such as wood, paper or ceramics, it may be desirable toseal the pores before applying the overlying electrode components. Themeans of providing such a sealing are well known and no furtherdiscussion of the same is necessary here.

According to a highly preferred embodiment of the present invention, thesupport comprises a sheet or film of an insulating polymeric material. Avariety of film-forming polymeric materials are well suited for thispurpose, such as, for example, cellulose acetate, poly(ethyleneterephthalate), polycarbonates, polystyrene, polyvinylchloride, etc. Thepolymeric support may be of any suitable thickness typically from about20-200 mils. Similarly thin layers or surfaces of other materialsmentioned above could be used. Methods for the formation of such layersare well known in the art.

Specifics of Enzyme Electrode

An enzyme sensor comprises a three-electrode system including a working,reference and counter electrode. The working electrode includes acomposite membrane that is deposited on a surface in contact with aconductive wire, a platinum wire for example. The composite membranecomprises two or more layers, including a enzyme layer and an innerinterference rejection membrane, for example.

The sensor fabrication may be based on solvent casting techniques wellknown in the art. The thickness of the layers can be controlled bydispensing precise volumes of solutes found in the layers. The polymericmembrane that comprises an inner interference rejection membrane,described in detail below, is formed onto the wire electrode byelectropolymerization of electropolymerizable monomers, as describedbelow.

Referring to FIGS. 3A and 3B, an enzyme electrode 59, such as a glucoseelectrode, is located in the flow channel 56 of the sensor card 50. FIG.3B is an enlarged section of FIG. 3A. The enzyme electrode 59 includes athree layer composite membrane 60 comprising, from the flow channel 56to the wire 57, an outer membrane 51 adjacent to the flow channel 56, anenzyme layer 53, located between the outer membrane 51 and an innerinterference rejection membrane 55 adjacent a wire 57. The enzymeelectrode 59 contacts the sample as the sample flows along the flowchannel 56 and over the outer membrane 51 of the enzyme electrode 59.The electrical signal generated by the enzyme electrode 59 is carried bythe wire 57 and transferred to the conductor 61 which is in electricalcommunication with the electrode assembly 10 shown in FIG. 2.

Referring still to FIGS. 3A and 3B, the outer membrane 51 of the enzymeelectrode 59 generally functions to control the diffusion of the analyteinto the enzyme layer 53 and to protect the other components of theelectrode 59 from direct contact with constituents of the sample inchannel 56. In one embodiment, the outer membrane 51 is a polymericmembrane comprising one or more polyurethane-based compounds. Thehydrophobicity of the membrane is determined by the mixture of speciesof polymer compounds. As the hydrophobicity of the membrane increases,the ability of oxygen to diffuse through the membrane increases whilethe ability of analytes to diffuse through the membrane decreases. Thepreferred composition of the outer membrane 51 is the concentration inwhich an optimal balance of diffusion rates of oxygen, which is arequired substrate of the enzymatic reactions, and analyte (lactate in alactate sensor, or creatinine and creatine in a creatinine sensor, andglucose in a glucose sensor) exists under typical conditions. A highlyhydrophobic outer membrane may be preferred because oxygen will diffusequickly to the enzyme layer 53 and thus will not be a limiting factor tothe enzymatic reaction. The outer membrane 51 may have a preferablethickness of 8 to 15 microns and could function with a thickness in therange of 5 to 30 microns.

The outer membrane 51 is composed of a blend of polyurethanes withdifferent water uptake levels. A typical composition of the outermembrane 51 is 77% aliphatic, polyether-based polyurethane with 20%water uptake, 17% aliphatic, polyether-based polyurethane with 60% wateruptake, and 6% aliphatic, polyether-based polyurethane with 3% wateruptake. The outer membrane 51 with this composition can be produced bydispensing a volume from a solution of 3.0 mL cyclohexanone solvent,17.0 mL tetrahydrofuran solvent, 1.08 g of 20% water uptakepolyurethane, 0.24 g as 60% water uptake polyurethane and 0.08 g as 3%water uptake polyurethane onto the enzyme layer 53 of the compositemembrane 60.

Referring to FIG. 3B, the outer membrane 51, which is layered directlyonto and in contact with the enzyme layer 53, functions to preserve theenzyme layer 53 by preventing exposure of an enzyme 49 embedded inenzyme layer 53, and the stabilizing matrix in which the enzyme 49 isembedded, to degradatory proteins or compounds from the sample inchannel 56. Likewise, outer membrane 51 prevents diffusion of the enzyme49 out of the enzyme layer 53. The outer membrane 51 also functions tocontrol the rate of diffusion of analyte (e.g. glucose, lactate,creatine and creatinine) and oxygen from the sample to the enzyme layer53. Failing to control the diffusion of the analyte and oxygen to theenzyme layer 53 results in non-linear and inaccurate measurements of theanalyte in the sample.

Referring still to FIG. 3B, the enzyme layer 53 of the glucose orlactate sensor, includes at least one enzyme 49 species required for theenzymatic reaction in which the specific analyte participates that isstabilized in the matrix of the enzyme layer 53. In one embodiment, theenzyme 49 includes at least one protein with enzymatic activity. Inother embodiments, enzyme 49 includes a mixture of several enzymes,proteins and stabilizers, for example.

In a particular embodiment of the invention, the protein enzyme 49glucose oxidase or lactate oxidase are embedded in the enzyme layer 53and create an electrode 91 and 92 specifically sensitive to glucose andlactate, respectively, present in the sample. The glucose electrode 91includes glutaraldehyde and glucose oxidase in the enzyme layer 53. Inone embodiment, the glucose electrode 91 may include 0.10 g ofglutaraldehyde per gram of glucose oxidase. In a particular embodiment,the lactate electrode 92 includes at least glutaraldehyde, bovine serumalbumin, a enzyme stablizer such as, for example, polyethyleneimine andlactate oxidase in the enzyme layer 53. In one embodiment, the lactateelectrode 92 includes 45% lactate oxidase by weight, 45% bovine serumalbumin by weight, 5% polyethylenimine (an enzyme stabilizer) by weightand 5% glutaraldehyde by weight, for example. The weight fractions oflactate oxidase and bovine serum albumin can vary. The weight percent ofpolyethylenimine in the enzyme layer can vary from 1 to 20, and theweight percent of glutaraldehyde can vary from 1 to 10. Other enzymesstabilizers include but are not limited to polyionic compounds such aspolypropyleneimine, poly(N-vinylimidazole), polyallylamine,polyvinylpiridine, polyvinylpyrollidone, polylysine, protamine and theirderivatives.

In yet another embodiment of the invention, enzyme layer 53 includes amixture of several enzymes, proteins, and stabilizers embedded in thematrix of enzyme layer 53 for specific detection of creatinine andcreatine or creatine only. Enzyme mixtures are used in the creatinineelectrode 116 and creatine electrode 118. Creatine alone is detectedwith the creatine electrode 118. In a particular embodiment, creatinineelectrode 116 includes a mixture of 5% creatininase by weight, 55%creatinase by weight, 30% sarcosine oxidase by weight, 5%poly(N-vinylimidazole) (an enzyme stabilizer) by weight and 5%glutaraldehyde by weight, for example. The weight fractions ofcreatininase, creatinase and sarcosine exidase in the creatinineelectrode and the weight fractions of creatinase and sarcosine oxidasein the creatine electrode can vary. The weight percent ofpoly(N-vinylimidazole) in creatinine and creatine electrodes can vary,for example, from 1% to 20%, and the weight percent of glutaraldehyde inthe creatinine and creatine electrodes can also vary, for example, from1% to 10%. Polyionic stabilizers, other than poly(N-vinylimidazole), canalso be used for stabilizing the enzyme mixture. Examples of polyioniccompounds include but are not limited to polyethylenimine,polypropyleneimine, polyallylamine, polyvinylpiridine,polyvinylpyrollidone, polylysine, protamine, and their derivatives.

In one embodiment of the glucose, lactate, creatine, and creatinineelectrodes, the enzyme layer 53 consists of a cross-linked matrix ofenzymes, stabilizers such as polyethylenimine or poly(N-vinylimidazole),and other proteins such as bovine serum albumin. Cross-linking of theenzymes, stabilizers, and other protein molecules is accomplished with,for example, glutaraldehyde, a dialdehyde. Other cross-linking reagents,such as 1,4-diisocyanatobutane, a diisocyanato, 1,2,7,8-diepoxyoctaneand 1,2,9,10-diepoxydecane, both diepoxides, can also be used.Cross-linking of the enzyme molecules and the use of the polyionicstabilizers and inert proteins in the enzyme matrix can significantlyextend the shelf-life and the use-life of the enzyme electrodes.

In yet another embodiment of the invention related to the creatinine 116and creatine 118 electrodes, enzyme layer 53 includes a mixture ofseveral enzymes, proteins, but lacks an enzyme stabilizer. In thisembodiment, the creatinine electrode 116 includes a mixture of 30%creatininase, 30% creatinase, 30% sarcosine oxidase and 10%glutaraldehyde (percentages by weight). In this embodiment, the creatineelectrode 118 includes a mixture of 45% creatinase, 45% sarcosineoxidase and 10% glutaraldehyde (percentages by weight). The enzyme layer53 may have a thickness in the range of 1 to 10 microns, preferably 2-5microns measured from the inner surface of the outer membrane 51 to theouter surface of the inner interference rejection membrane 55.

Referring to FIGS. 3A and 3B, the enzyme electrode 59 also includes aninner interference rejection membrane 55 which is a restorable polymericmembrane in close contact to the wire 57. The inner interferencerejection membrane 55 may be formed by the polymerization ofelectropolymerizable monomers. Suitable electropolymerizable monomersinclude benzothiophene, phenylenediamines, and phenols, for example. Theinner interference rejection membrane 55, which is typically less than amicron thick, insulates or protects the wire 57 from compounds in thesample, specifically oxidizable compounds, that interfere with theproper functioning of the enzyme electrode.

In one embodiment according to the invention, the polymeric membranecomprising the inner interference rejection membrane 55 is formed by theapplication of an electrical potential to the wire 57 in the presence ofelectropolymerizable monomers. The monomers in the presence of anelectrical potential polymerize on the wire 57 to form an electricallyinsulating polymeric inner interference rejection membrane 55 on thewire 57 illustrated in FIGS. 3A and 3B. Hydrogen peroxide, which isgenerated from activity of the enzyme of the enzyme electrode on aspecific analyte, passes through the pores of the inner interferencerejection membrane 55 and contacts the wire 57 causing an electricalsignal to be generated at the wire 57. The smaller size of the pores inthe inner interference rejection membrane 55 restricts compounds foundin the sample, larger than hydrogen peroxide, such as acetaminophen,ascorbic acid, uric acid, cysteine and other electroactive compoundsthat are larger than H₂O₂ from interfering with and reducing accuracy ofthe electrode 59 of the electrochemical sensor.

According to one embodiment of the invention, the inner interferencerejection membrane 55 may be regenerated on a repeated basis to restoreits function. Following repeated exposure to many samples, the innerinterference rejection membrane 55 is degraded or fouled by compoundspresent in the sample. Degradation of the inner interference rejectionmembrane 55 is characterized by fissures in the polymeric structure ofthe inner interference rejection membrane 55. Such fissures prevent theability of the inner interference rejection membrane 55 to protect thewire 57 from interfering compounds present in the analytical sample,e.g., ascorbic acid, acetaminophen, and uric acid, from contacting thewire 57 and altering the electrical signal detected by the wire 57.

In order to avoid problems induced by the degradation of the innerinterference rejection membrane 55, an electropolymerizable monomer canbe combined with a calibration solution, such as solution AO containedin the prepackaged container 23 of the electrochemical sensor system 8illustrated in FIG. 1, for example, for use in the repolymerization andrestoration of the inner interference rejection membrane 55.Polymerization of the monomer occurs when a monomer-containing AOsolution is pumped from a prepackaged container and passed through theflow channel 56 on the sensor card 50 during the application of anelectrical potential generated by electrochemical sensor apparatus 8illustrated in FIG. 1 to the wire 57. During the polymerization processthe monomer in the calibration solution in flow channel 56 diffusesthrough the outer membrane 51 and enzyme layer 53 until reaching theinner interference rejection membrane 55. Once at the inner interferencerejection membrane 55 the monomers present in the solution enter theareas of the inner interference rejection membrane 55 that have loststructural integrity by degradation, splitting or cracking, and mediatethe restoration of the inner interference rejection membrane 55 bypolymerizing to fill in the damaged structure of the inner interferencerejection membrane 55. The monomer is exposed to an electrical potentialgenerated from an electrical source and transferred to the wire 57 inthe areas of lost integrity of the inner interference rejection membrane55. The electrical potential polymerizes the monomer onto the existingpolymeric structure of the inner interference rejection membrane 55 atdamaged areas of the inner interference rejection membrane 55 until theinner interference rejection membrane 55 is restored. Once the innerinterference rejection membrane 55 is restored the insulating propertiesof the inner interference rejection membrane 55 is renewed and themonomer present at the inner interference rejection membrane 55 issequestered from the electrical potential of the wire 57. Thisself-limiting restoration of the inner interference rejection membrane55 is automatically repeated every 24 hours, for example. Regular,automatic, self-limiting restoration of the inner interference rejectionmembrane 55 ensures accuracy of the enzyme sensor 59. More or lessfrequent restoration cycles of the inner interference rejection membrane55 can be employed to account for different situations.

The electrical potential for the polymerization process generated by theelectrochemical sensor system 8 illustrated in FIG. 1 is applied to thewire 57 in the range of 0.1 to 0.8 V versus the on-board referenceelectrode 106, for about 30 seconds to one hour. An optimal polarizationpotential is 0.5 V versus the on-board reference electrode 106 for 3minutes, repeated every 24 hours. The electrical potential is too low ifit does not cause the polymerization reaction and the electricalpotential is too high if it causes water hydrolysis and gas formation atthe inner interference rejection membrane 55 thus causing damage to theenzyme electrode 59.

Specifics of the PO₂ Electrode

An oxygen sensor comprises a three electrode system including a workingelectrode, a reference electrode and a ground electrode. In oneembodiment of the invention, the oxygen working electrode 70 comprises aplatinum wire 74 that is fixed in the center of an insulative glass disk109 and two protective membranes 120 and 122 best shown in FIG. 4. Thedisk preferably has a thickness of approximately 40 mils while the board50 may have a thickness of approximately 85 mils. The diameter of theglass disk is preferably about 100 mils.

A number of the glass disks with the embedded platinum wires areprepared by inserting a close-fitting length of platinum wire into thelumen of a glass capillary tube and then melting the tube so that itfuses to the wire. After the tube with the embedded wire hardens, thedisks of given axial thickness are sliced off, by a power saw, forexample.

The glass disk is practically impervious to oxygen whereas thepolyvinylchloride of the board 50 is relatively pervious. The glass diskthus protects the platinum electrode 74 from the gas so that only itsdistal end that faces the flow channel 56 is active.

The two membranes 120 and 122 on the glass disk protect the platinumwire 74 from direct contact with the constituents of the sample inchannel 56. In one embodiment, the membrane 120 is a hydrogel based onmethacrylic esters that is covalently bonded to the glass disk. Themembrane 122 underneath the 120 covers only the area around the platinumwire and is made of polyvinyl alcohol. The composite membrane 60including 120 and 122 provides a better protection and sensorperformance than either of the membranes alone. The type of hydrogelthat is employed is based on methacrylic esters, although hydrogels notbased on esters of methacrylic acid may be used. To form a gel, themonomer, such as hydroxyethyl methacrylate or hydroxypropylmethacrylate, for example, is copolymerized with a cross-linker, suchas, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate ortetraethylene glycol dimethacrylate. The cross-linking reaction can beinitiated by a photoinitiator such as dimethoxyphenylacetophenone. Asolvent such as ethylene glycol or water can be used to dilute reactionsand control the viscosity of the solution.

It is of considerable advantage that the hydrogel membrane does not peelaway from the surface of the oxygen electrode when the membranehydrates. This is achieved by functionalization of the glass disk withmethacrylic groups and cross-link the membrane to the surface. Thesurface of the glass disk is silinized with hexamethyldisilazane andfunctionalized with methacrylic groups by reacting with trimethoxysilylpropyl methacrylate.

After functionalization of the glass disk, a small drop of a solution ofpolyvinyl alcohol in water is dispensed at the center of the diskdirectly over the platinum wire and the water is allowed to evaporatefor formation of the polyvinyl alcohol membrane. A solution of hydrogelcomponent as described above is then dispensed on the disk in an amountcorresponding to 50-micron thick film. The disk is exposed to a broadband UV light for 5 min to photopolymerized the hydrogel membrane.

The glass disk with the composite membrane on one side of it is embeddedin a recessed form through the thickness of the plastic board 50 so thatthe non-hydrogel surface is flush with the surface of the board oppositethe cover plate 52 and the hydrogel surface of the disk is flush withthe bottom of the flow channel 56.

The oxygen sensor described here has several advantages when compared tothe conventional electrode (Clark electrode), including smallerelectrode size, simpler electrode fabrication, faster response time andlonger use life. Separation of the reference and the counter electrodesform the working electrode allows for smaller size of the workingelectrode and simpler electrode fabrication. The oxygen response time isreduced because of the absence of internal solution and the resultingthinner membrane over the working electrode. The use of externalreference electrode eliminates the silver dendrite formation on theworking electrode, which is a common mode of failure in a Clark oxygenelectrode with an internal Ag/AgCl reference electrode.

Concerning the amperometric function of the electrode in operation, anegative potential relative to the on-board reference electrode 106 isapplied to the platinum wire 74 by the processor 40 which lessenedpotential serves to reduce any oxygen reaching its end and therebyproduces an electrical current proportional to the oxygen diffusionthrough the layers 120 and 122 The hydrated layer 120 and 122 affords areliable conductive flow path between the platinum electrode and theon-board reference electrode 106 to provide a polarization potentialbetween the platinum and the solution in the hydrated layer. Theresulting current flow between the platinum electrode 74 and the groundelectrode is measured and is proportional to the oxygen concentration inthe test fluid being monitored.

pCO₂, pH, Potassium, Sodium and Calcium Sensing Electrodes

The electrodes, best illustrated generally in FIG. 2, connecting thesilver wires 78, 86, 90, 93, and 94 which sense Na, Ca, potassium, pCO₂and pH activities, respectively, are similar in construction. Thedifference is in the composition of the membrane layers. A typicalion-selective electrode is illustrated in FIG. 6. Each has a bead or aninner salt layer 152, which upon hydration forms the inner solutionlayer. This layer is in contact with the thin film of silver/silverchloride layer 154 obtained by anodization of the top of the silverwires. The outer layer 148 is essentially the polymeric ion-selectivemembrane layer. This layer is formed over the dried salt residue of theinner layer in a shallow well 150 as a dry residue remaining after thesolvent removal from a matrix of a permeable hydrophobic membraneforming solution such as a solution containing polyvinylchloride, aplasticizer, an appropriate ion-sensing active ingredient and a boratesalt. The outer membrane is applied as a solution, typically inTetrahydrofuran (THF) in a small droplet. Once the solvent evaporates,the membrane is formed and is bonded to the plastic card. In the case ofpH and pCO₂ electrodes, the ion-selective active ingredient may betridodecylamine (TDDA) or a suitable pH sensing component. For thepotassium electrode, a monocyclic antibiotic such as valinomycin may beused as the active ingredient. The calcium electrode employs a calciumion-selective sensing component as its active ingredient such as(−)-(R,R)-N,N′-(Bis(11-ethoxycarbonyl)undecyl)-N,N′-4,5-tetramethyl-3,6-dioxaoctanediamide;DiethylN,N′-[(4R,5R)-4,5-dimethyl-1,8-dioxo-3,6-dioxaoctamethylene]-bis(12-methylamino-dodecanoate)or other suitable calcium sensitive selective substance. The sodiumelectrode employs methyl monensin ester or any other suitable sodiumsensitive active ingredient. The sodium, potassium and calciumelectrodes use a buffer salt like MES (2-[N-morpholino]ethanesulphonicacid) along with the respective chloride salts for their inner solution.

pH and pCO₂ electrodes share the same outer layers, while their innerlayers differ significantly. The internal layer for pH uses a strongbuffer, for example, MES buffer, while that for CO₂ electrode use abicarbonate buffer.

All ion-selective electrodes, except CO₂ electrode, operate through themeasurement of the potential between the ion-selective electrode and thereference electrode 106 (FIG. 2), the change in potential is directlyproportional to the change in the logarithm of the activity of themeasured ion.

The CO₂ sensor is a combination of CO₂ and pH electrodes workingtogether. In function the potential between the CO₂ and pH electrode ismeasured. The outer surface of both electrodes respond to pH in the samemanner and cancel each other. The inner surface of the pH membrane has ahigh buffer with constant pH and does not cause any change in themeasured potential. However, for CO₂, the membrane is freely permeableto CO₂, which dissolves in the bicarbonate buffer changing its pH. Thiscauses a change in the potential response of the inner surface of theCO₂ membrane, which is the only change to the Overall measuredpotential. Thus, the potential across the CO₂ and pH electrodes directlymeasures the variation in the CO₂ concentrations of the sample.

The process of hydrating the inner salt layer in these ion-selectiveelectrodes is achieved by soaking the outer surface of the outermembranes in an aqueous salt solution, usually a calibrating reagentsolution. The hydration, however, is a very slow process, as the waterhas to permeate through the hydrophobic outer membrane in the vaporform. Thermal cycling through high temperatures facilitates the process.During the process of thermal cycling, the composition and integrity ofthe membrane layers stay intact.

Hydration and calibration of the ion sensing electrodes are accomplishedby steps similar to those described for the pO₂ electrode. Hydrationfrom a dry state can be accelerated by soaking the sensors in anelectrolyte solution, such as the calibrating solutions described above,and thermally cycling the sensors through an elevated temperature higherthan that of normal use. For example, the sensors are soaked incalibrating solution B at a temperature between 55° C. to 75° C. for 15minutes, and then cooled to 37° C. The calibration cycles start as soonas the temperature reaches 37° C. In a preferred embodiment, the sensorsare soaked in a calibrating solution at a temperature of 60° C. for 12minutes, and then cooled to 37° C. The calibration cycles start as soonas the temperature returns to 37° C.

Hematocrit Measurement

The hematocrit (Hct) measurement is made through a measurement ofresistivity between gold wires 98 and 100. The sensor operates bymeasuring the resistivity of the solution or blood sample placed betweenthe electrodes. Hematocrit is calculated as a function of resistivityusing the Maxwell equation.

Removal of Interfering Agents

Exposure of the enzyme electrode 59 to the sample in the flow channel 56causes the composite membrane 60 to retain residual concentrations ofsubstrate from the sample and products of the enzymatic reaction fromthe operation of the enzyme electrode 59. These substances are examplesof the interfering agents that will cause the enzyme electrode 59 tolose accuracy and precision in measurement of the specifically intendedanalyte. In order to restore accuracy and precision to the enzymeelectrode 59, interfering agents are removed from the composite membrane60 of the enzyme electrode 59 by applying an additional amplitude ofpolarization to the wire 57 of the enzyme electrode 59.

A polarization pulse may be applied by an electrical source to the wire57 after each exposure of the electrode 59 to a sample in order toprepare the electrode 59 for the next measurement. For example, hydrogenperoxide, a product of the reaction of the enzyme and the analyte fromthe operation of the electrode 59, is an example of an interferingagent. To remove interfering agents such as hydrogen peroxide, anadditional amplitude of polarization is applied to the wire 57 whichcauses oxidation of the interfering agent. Oxidation of the interferingagent renders the interfering agent incapable of affecting theelectrical activity at the wire 57 by effectively removing the agentsfrom the electrode 59. The analytes, such as glucose and lactate, alsoconstitute interfering agents when residual concentrations of glucoseand lactate remain in the enzyme electrode 59 between sample readings. Apolarization pulse applied to the wire 57 oxidizes the residual analyteand thus eliminates contribution of residual analyte between samples toerroneous analyte measurements.

In one embodiment according to the invention, after measurement of ananalyte in a sample is complete, the enzyme electrode 59 is restored bypumping the sample out of the flow channel 56, and a volume of washsolution from reservoir 17 is pumped through the flow channel 56. Duringthis time, an additional polarization is superimposed on the stablepolarization continuously applied to the electrodes 59 after a samplemeasurement. The polarization is then returned to its baseline level anda calibration solution is introduced into the flow channel 56 followedby a one-point calibration to ready the electrode 59 for the nextmeasurement.

The sufficient amplitude and duration of the polarization pulse requiredfor the oxidation of interfering agent is determined by the geometry ofthe flow channel 56. Greater pulse amplitudes and longer pulse durationsare required for an electrode 59 with a narrow flow channel 56 and aslow flow rate of wash solution. In a preferred embodiment illustratedin FIG. 3A, a polarization amplitude of 0.4 V versus the on-boardreference electrode for a duration of 50 seconds is sufficient toeliminate interfering compounds from the composite membrane 60, and thusimprove accuracy and precision of the electrode 59 measurements. Apolarization amplitude in the range from 0.1 to 0.8 V versus theon-board reference electrode for a duration of 10 to 200 seconds mayalso be sufficient.

Restoration of the Inner (Interference Rejection) Membrane of theComposite Membrane

A further step to restore the function of the inner interferencerejection membrane 55 of the composite membrane 60 of the enzyme sensor59 illustrated, for example, in FIG. 3B. This step includes restorationof the integrity and proper functioning of the inner interferencerejection membrane 55 of the enzyme electrodes. Within the compositemembrane 60, illustrated in FIG. 3B restoration of the innerinterference rejection membrane 55 occurs by the in situ polymerizationof electropolymerizable monomers onto the inner interference rejectionmembrane 55 of the composite membrane 60.

In one embodiment, the electropolymerizable monomers are in solution inthe AO calibration solution in container 23 illustrated in FIG. 1 The AOcalibration solution is passed through the flow channel 56 of the sensorcard 50 illustrated in FIG. 3A. The AO solution with theelectropolymerizable monomers contact the enzyme electrode 59 at theouter polymeric membrane 51 of the composite membrane 60. Theelectropolymerizable monomers diffuse first through the outer membrane51, and then through the enzyme layer 53 of the composite membrane 60,until the monomers reach the inner interference rejection membrane 55 ofthe composite membrane 60. An electrical potential greater thanbaseline, 0.5 V versus the on-board reference electrode for is appliedto the wire 57 for 3 minutes, for example, causing theelectropolymerizable monomers to polymerize onto the existing polymericstructure of inner interference rejection membrane 55 of the compositemembrane 60. Following the polymerization of the inner interferencerejection membrane 55, the insulating properties of the innerinterference rejection membrane 55 are restored. Because the remainingelectropolymerizable monomers in the calibration solution are no longerexposed to the electrical potential, polymerization of the monomers canno longer occur.

The amplitude of the electrical potential and the period of time of theelevated potential sufficient for restoration of the inner interferencerejection membrane 55 of the composite membrane 60 is determined by thespecific configuration of the electrode 59. The composition and theparticular geometry of electrode affects the amplitude of the electricalpotential and the period of time required for complete restoration ofthe inner interference rejection membrane 55. A composite membrane 60 ofa composition or geometry that slows the diffusion of monomers from theflow channel 56 to the inner interference rejection membrane 55 willrequire a greater polymerization amplitude for a greater duration oftime. A polarization of about 0.1 to 0.8 V versus an on-board referenceelectrode applied for about 30 seconds to 1 hour is suitable for atleast partial restoration of the inner interference rejection membrane55. Once the restoration of the inner interference rejection membrane 55is complete, the AO solution in flow channel 56 is replaced with rinsesolution 17 and the electrical potential is returned to baseline.

Reference Solution Operation

Referring to FIG. 2, as has been noted, the reference solution fills thewell 64 where it contacts a silver wire 106 and is pumped through thecapillary channel 66 to join the outlet of the main flow line. Thereference solution is essentially a hypertonic solution of potassiumnitrate, with respect to the blood or the calibrating solutions andaccordingly the domain of the reference electrode 106 constitutes astable potential liquid junction formed between the reference electrodeand the blood or calibrating solution, thereby establishing anenvironment that is independent of the ionic activity of the blood orcalibrating solution.

Since the reference solution joins the main flow channel downstream fromthe electrodes, it does not affect those measurements in any way. Thereference solution is of high density and under pumping force must flowupward against gravity to the outlet. Thus, when the pump stops, as forelectrode equilibration, the reference solution remains stationary inthe reference well 64 and the capillary section 66 and tends not todiffuse into the calibrating solution or blood in the main flow channel.Thus, the capillary tube 66 due to the density gradient, acts as a oneway valve allowing pumped reference solution to pass upwardly throughthe capillary but preventing unwanted reverse passage or mixing of theblood or calibrating solution into the reference well.

Heater Block Assembly

Referring to FIGS. 7A-7G, the heater block assembly 39 includes athermoelectric device 230, a thermistor 41, an aluminum block featuringtwo aluminum shells 220 a, 220 b, electrode interface 156, metal plate234, heat sink 236, electrical leads 229, 229′, 231, 231′, and cable226. The aluminum block houses a sensor card 10 when the cartridge withthe sensor card is inserted into the fluid analysis instrument 8.

Referring to FIG. 7A, the aluminum heater block assembly 39 includes twoaluminum shells 220 a, 220 b which together form a socket 222 into whicha sensor card 10 (not shown) can be inserted. As illustrated in FIG. 7B,electrical connection 156 located in socket 222, interfaces with thecorresponding edge connectors in the sensor card illustrated in, forexample, FIG. 5, to transmit signals from the sensors. A cable 226connects the electrical connectors from the sensor card to amicroprocessor 40 through an analog board 45 (See FIG. 1). A printedcircuit board (analog board located before the processor) controls thesensors and measures sensor output. Printed circuit boards within thisheater block assembly contain post amplifiers that amplify signals fromthe sensor in the sensor card. The output of the sensors are analogsignals. The analog signals are converted to digital signals via ananalog to digital converter, and the digital signals are transmitted tothe microprocessor for storage, analysis, and display.

Referring to FIG. 7C, the interior surface 221 of aluminum shell 220 bcomes into contact with the metal plate 52 of a sensor cartridge 10 (seeFIG. 2). On the external surface 223 of aluminum shell 220 b, athermistor 41 is located as illustrated in FIG. 7C. Extending fromthermistor 41 are electrical connections 229, 229′ that connect thethermistor 41 to a microprocessor 40.

On top of the external surface 223 of aluminum shell 220 b and over thethermistor 41, a thermoelectric device 230 illustrated in FIG. 7D, ispositioned. Thermoelectric devices in the heater block assembly may use,for example, the Peltier-effect, to heat and cool the aluminum block.Electrical leads 231, 231′ supply programmed electrical currentcontrolled by a microprocessor 40 to the thermoelectric device 230. Thedirection and duration of current is controlled by the microprocessor 40and determines whether the thermoelectric device 230 overlying thealuminum shell 220 b is in a warming or cooling mode. The temperature ofthe aluminum shell 220 b is measured by thermistor 41 which transmitssignals to microprocessor 40. Microprocessor 40 is programmed totransmit electrical signals to the thermoelectric device, depending onsignals from the thermistor, to either heat or cool the aluminum shell220 b which in turn heats, cools or maintains the temperature of asensor card inserted into socket 222. When current flows in thethermoelectric device 230 in the forward direction, the metal plate 220b is heated and this heat is transmitted to the sensor card in thesocket 222. When current flows in the reverse direction, the metal plate220 b is cooled and the cooling effect is transmitted to the sensor cardin the socket 222.

Referring to FIGS. 7D and 7E, the external surface 233 of thethermoelectric device 230 is in contact with a metal plate 234. Theexternal surface 235 of metal plate 234 is in contact with a heat sink236, illustrated in FIG. 7F.

The assembled cartridge socket 222, aluminum shell 220 b, thermistor 41,thermoelectric device 230, metal plate 234, heat sink 236 and electricalleads 229, 229′ from the thermistor 41, and electrical leads 231, 231′from the thermoelectric device 230 to the microprocessor 40 isillustrated in FIG. 7G.

In a preferred embodiment of the heater block assembly 39, thetemperature for a sensor cartridge can be increased from about 37° C. toabout 60° C. to 65° C. in one minute, maintained at 60° C. for 12minutes with only 1.0° C. temperature fluctuation, and cooled to 37° C.from 60° C. in about two minutes.

Initial Operation of the Assembly

Referring to FIG. 1, when the cartridge with the sensor assembly 10 andthe filled bags 14, 16 and 28 are first used, the valve 18 is controlledto direct one of the calibration solutions for example, calibrationsolution B, into the sensor assembly so it entirely fills the flowchannel. The pump is then stopped for a period of 10-30 minutes,preferably 12-15 minutes during which the dry chemical sensor electrodesare hydrated by thermal cycling, for example, from 37° C. to 60° C. andback to 37° C.

In one embodiment of the invention, the dry chemical electrode sensorassembly 10 is inserted into the electrochemical sensor system 8 and thevalve 18 is controlled by microprocessor 40 to direct the calibrationsolutions B into the sensor assembly 10. Thermal block assembly 39 isset at a temperature whereby the temperature of thermal plate 52 issufficient to heat the calibrating solution in contact with the drychemical sensor to a temperature in a range of 55° C. to 75° C.,preferably 60° C., for 10-30 minutes, preferably 12 minutes. After thespecified time period, the microprocessor 40 reverses current flowthrough the thermoelectric device to cool thermal plate 52. The sensorcard 50 and calibrating solution in contact with thermal plate 52 arecooled to 37° C. The temperature, controlled by the microprocessor 40,is maintained at 37° C. for the life of the cartridge 37. Afterhydration of the sensors, the conditioning cycle of the enzymeelectrodes 59 starts by pumping the AO solution 23 to the sensor card 50and soaking the electrodes 59 for 1 to 6 minutes, preferably, for 3minutes while the polarization potential of the enzyme electrodes 59 iselevated from 0.25 to 0.5 V versus the on-board reference electrode.During the AO exposure, the inner interference rejection membrane 55 ofthe enzyme electrodes 59, illustrated in FIG. 3B, is restored. Moreover,in this cycle the low oxygen level is also calibrated. Upon completionof the AO cycle, the rinse cycle starts by pumping rinse solution fromprepackaged container 17 the flow channel 56 by the peristalic pump 26.During the rinse cycle the polarization potential of the enzymeelectrodes 59 is changed from 0.5 to 0.4 V in order to accelerate theremoval of the AO residues from the inner interference rejectionmembrane 55. Following the completion of the rinse cycle, thepolarization potential of the enzyme electrodes 59 are lowered back tonormal level of about 0.25 V versus the on-board reference electrode. Acalibration cycle with solutions A 14 and B 16 then begins. Thecartridge 37 becomes ready for sample measurement within 30 minutes ofcartridge 37 insertion into the electrochemical sensor system 8.

1. A composite membrane for an enzyme sensor comprising: an innermembrane layer comprising a restorable polymerizable membrane, the innermembrane layer adapted to function as an interference rejectionmembrane; an outer membrane layer comprising polyurethane, and adaptedto be disposed adjacent a flow channel during use; and an enzyme layerdisposed between and in contact with the inner and the outer membranelayers, said enzyme layer comprising a matrix comprising: at least oneenzyme selected from the group consisting of glucose oxidase and lactateoxidase; a cross-linking agent; and an enzyme stabilizer selected fromthe group of polyionic compounds consisting of polyethyleneimine,polypropyleneimine, poly(N-vinylimidazole), polyallylamine,polyvinylpyridine, polyvinylpyrrolidone, polylysine, and protamine;wherein the outer membrane is adapted to control diffusion of an analyteinto the enzyme layer.
 2. The composite membrane of claim 1, whereinsaid enzyme sensor comprises an electrochemical electrode.
 3. Thecomposite membrane of claim 1, wherein the enzyme layer comprises aninert protein.
 4. The composite membrane of claim 1, wherein saidcross-linking agent is selected from the group consisting of adialdehyde, a diisocyanate, and a diepoxide.
 5. The composite membraneof claim 4, wherein said cross-linking agent comprises glutaraldehyde.6. The composite membrane of claim 4, wherein said cross-linking agentcomprises 1,4-diisocyanatobutane.
 7. The composite membrane of claim 4,wherein said cross-linking agent is selected from the group consistingof 1,2,7,8-diepoxyoctane and 1,2,9,10-diepoxydecane.
 8. The compositemembrane of claim 1, wherein the enzyme layer has a thickness in therange of about 1 micron to about 10 microns.
 9. The composite membraneof claim 1, wherein the enzyme layer has a thickness in the range ofabout 2 microns to about 5 microns.
 10. The composite membrane of claim1, wherein the outer membrane layer comprises a blend of polyurethaneshaving different water uptake levels.
 11. The composite membrane ofclaim 1, wherein the outer membrane layer has a thickness between about8 microns and about 15 microns.
 12. The composite membrane of claim 1,wherein the enzyme stabilizer is polyethyleneimine orpoly(N-vinylimidazole), and comprises a weight percent of the enzymelayer that is between about 1% and about 20%.
 13. The composite membraneof claim 1, wherein the enzyme stabilizer is polyethyleneimine.
 14. Thecomposite membrane of claim 1, wherein the enzyme stabilizer ispoly(N-vinylimidazole).
 15. The composite membrane of claim 1, whereinthe enzyme layer comprises lactate oxidase, glutaraldehyde, bovine serumalbumin, and polyethyleneimine.
 16. The composite membrane of claim 1,wherein the restorable polymerizable membrane is formed by thepolymerization of electropolymerizable monomers selected from the groupconsisting of benzothiophene, phenylenediamines, and phenols.